Microelectromechanical Systems (MEMS) devices are typically attached to a housing structure, and the housing structure attached to another entity, such as a circuit board. Small strain forces between the housing structure and the MEMS device may detrimentally affect the performance of the MEMS sensor located within the MEMS device.
It is common practice to attach a MEMs die to a housing substrate using the full surface of the handle layer (the underside of the MEMs die). MEMs devices are sensitive to packaging strains which may be caused by a mismatch in Thermal Coefficient Of Expansion (TCE) and by the elevated bonding temperatures involved during the attachment process. For example, FIG. 1A illustrates conceptually the shape of a MEMS die 10 at bonding temperature relative to a layer of bonding agent 12, such as epoxy, used to attach the handle layer of the MEMS die 10 to a substrate surface 14. FIG. 1B illustrates the deep formation of the MEMS die shape after bonding, at room temperature, as a result of epoxy shrinkage and TOE mismatch between the material of the MEMS die 10 and the bonding agent 12. Such deformations create strain forces may have an influence on the accuracy and performance of the sensor, particularly sensors such as gyroscopes and accelerometers, and even more specifically for sensors implemented with bulk acoustic wave resonators in which the distances and alignment between the resident body and the drive and sense electrodes are microscopic.
Accordingly, need exists for a way to reduce strains on a MEMS die caused by the manner and process by which MEMS die is mounted.
An additional concern created by the practice of attaching a MEMs die to a housing substrate using the full surface of the handle layer, is the increased sensitivity to external forces transmitted through the package housing. Such configuration allows for any shock, vibration or other external force on the package to be mechanically coupled to the MEMs die and, ultimately, to any sensor mounted on the MEMs die. Again, such forces may influence the accuracy and performance of the sensor, particularly sensors such as gyroscopes and accelerometers. Prior attempts have been made to address the issue of isolating a MEMS die for mechanical vibrations. For example, U.S. Pat. No. 7,280,262 discloses a technique for attaching a MEMS device to its packaging at only the edges of MEMS die, however, although such device reduces the area of the handle layer surface secured to a housing surface, external forces and vibrations may still be transmitted to the sensor on the MEMS device from the edges of the MEMS die which is particularly problematic with a bulk acoustic wave sensor having a disc shaped resonant body.
Accordingly, a need exists for a mechanism to attach a MEMS die to its respective packaging in a manner which reduces external influences, such as mechanical vibrations incident on the MEMS device packaging.